The present invention relates to a brake system for an automobile. More specifically, the present invention relates to a master cylinder apparatus used in a brake fluid pressure controlling system for electrically controlling a fluid pressure supplied to wheel cylinders; i.e., a so-called brake-by-wire (BBW) system.
A master cylinder apparatus for a BBW system comprises a master cylinder adapted to be connected to wheel cylinders through fail-safe valves and a stroke simulator for ensuring a desired stroke of a brake pedal upon receiving a brake fluid introduced from the master cylinder. In the event of failure of the BBW system, the fail-safe valves are opened, and the fluid pressure generated in the master cylinder is supplied to the wheel cylinders.
This type of related art is disclosed in, for example, U.S. Pat. No. 6,192,685 B1.
However, the master cylinder apparatus of U.S. Pat. No. 6,192,685 B1 is configured such that supply of brake fluid from a reservoir to a primary-side pressure chamber and a secondary-side pressure chamber is controlled by means of a center valve. Consequently, axial dimensions of the primary piston and the secondary piston are large, and an overall size of a master cylinder is also large.
To counter this problem, it has been suggested to use a plunger-type master cylinder in which cup seals are provided on an inner surface of a bore of a cylinder body, so as to seal outer circumferential surfaces of a primary piston and a secondary piston, each of which is subject to a sliding movement in the cylinder body. In the case of a master cylinder apparatus for a BBW system, it is generally required to set a spring force of a return spring for biasing a secondary piston to be larger than that of a return spring for biasing a primary piston, so as to limit a force that a driver is required to apply to a brake pedal. In such a configuration, since a spring force for biasing the secondary piston must be made large, and a return position (a retracted position) of the secondary piston during non-braking is therefore variable, a possibility exists that in the event of a failure of the BBW system, an invalid stroke for operating the secondary piston will be large, resulting in a delay in braking.